1. Field of the Invention
This invention relates in general to apparatus for the recording and playback of signals, and in particular to recording and playback apparatus in which intermodulation between signals in adjacently recorded tracks is minimized and/or precluded.
2. Description Relative to the Prior Art
There are many recording applications in which signals are recorded in side-by-side tracks of a recording medium. The respective bits of a digital word, for example, are often recorded in parallel, side-by-side, tracks of a magnetic tape or disc. In multitrack longitudinal magnetic recording of analog signals, recordings are also made in side-by-side tracks. To assure against crosstalk between signals recorded in a plurality of adjacent tracks, it is usual to employ unrecorded regions, i.e. guardbands, between such tracks. This is, naturally, wasteful of the recording surface and, in applications involving a large number of guardband-separated tracks, the recording medium that is required may be so wide as to render the using hardware non-competitive in a commercial sense. There is, therefore, a strong desire to obviate the use of guardbands.
As is known, there is, also, a strong trend toward the recording of video signals by use of a transducer such as a multitrack scanning magnetic head, whereby the requisite recording bandwidth may be reduced in proportion to the number of tracks employed. (A lessened bandwidth means that low recording tape speed and simplified hardware may be utilized for the recording operation.) When video recording with a multitrack scanning head, each video line is, for example, broken into the same number of picture elements (hereinafter called pixels) and common pixels of the respective video lines are recorded in the same track by the same core of the multitrack head. Thus, the number one pixel of all video lines 1 through 525 (assuming a 525 line video frame) is, for example, recorded in track number one by core number one; the number two pixel of all video lines 1 through 525 is recorded in track number two by core number two; etc. Assuming there are, say, 256 pixels per video line, there are, in the above-described recording system, 255 guardbands that it would be desirable to do without.
Signals corresponding to the pixels of a video frame (video frames occur, typically, at 30 per sec.) are produced, usually, by sampling the corresponding video signal at the pixel rate, i.e. at 256.times.525 per thirtieth of a second, or at about 4.times.10.sup.6 per second. Such sampling, as is known, per Nyquist, will provide unaliased spatial frequencies in the video display to about 2 MHz.
In a signal sampling system, it is well known to convert signal samples (which are generally rectangularly shaped impulses) to (sin x)/x (or similar) waveforms and to provide that successive signal samples in a given signal channel temporally occur at the "zero crossings" of the waveforms of other signal waveforms, thereby to avoid intersymbol interference. U.S. Pat. No. 2,952,745, as well as Transmission Systems For Communications, Bell Telephone Laboratories, Inc., 1964, and other literature, disclose such practice. Whereas the technique of preshaping pulses has been employed to avoid interference between pulses in a single channel, a commonly employed technique for providing "guardbandless" track-to-track isolation between adjacent tracks has been by skewing the signal information of adjacent tracks with respect to each other. Guardbandless Sony Betamax video recordings, for example, resemble herringbones, with each half of any given "herringbone" constituting a discrete video track. Such a technique provides good, but not total, isolation between the signals of adjacent tracks; and any skewing of the tape during playback will cause a decrease of wanted signals at the expense of unwanted (adjacent) signals.
Recordings such as are made with Sony Betamax recorders are serial in nature, i.e. first one track is recorded; then another; and so on. As noted above, however, there are instances when a plurality of bits, as in a digital word, simultaneously appear for recording in respective tracks. To effect core-to-core gap skewing in a multitrack head to provide track-to-track isolation in the manner of the Betamax machine would be nigh impossible, since no multitrack head with alternatingly skewed core gaps is known. Thus, a technique for providing track-to-track isolation, regardless of whether the signal information occurs serially or in parallel, is desirable.
Aside from the matter of tape track-to-track isolation--which manifests itself as a playback phenomenon--it is also desirable to provide isolation between signals processed through a magnetic record head. It is for this reason that shields are usually placed between the cores of a multitrack magnetic head. Without such shields, flux in one core can spread to an adjacent core and, in bridging the gap of the adjacent core, effect unwanted recording in the track that corresponds to the adjacent core.